Touma could completely descend into the underworld if there were a rift between the two worlds. If the rift is natural (in certain places), Imagine Breaker will not cancel it. If it's not natural, Touma will still pass through, but Imagine Breaker will disrupt his return path. Alternatively, using phase-shifting magic (like Aleister's) will pull Touma into the underworld. Imagine Breaker will have no effect on demons in the underworld (their place of origin) but will still work against magic. If any human or creature not a demon descends into the underworld through means other than summoning (not through teleportation or physically descending), they will be ejected back to their own world.

Is it too early, do you think? And he can find Scion's whereabouts and observe the multiverse if he wants. It has been demonstrated in some Collab where Kamachi himself wrote. Although it doesn't make him stronger, he can find a way there if he tries his best.